The present invention relates to optical fiber communication systems, and more particularly to an improved rare earth-doped optical fiber amplifier.
Cable television systems currently distribute television program signals via coaxial cable, typically arranged in tree and branch networks. Coaxial cable distribution systems require a large number of high bandwidth electrical amplifiers. For example, forty or so amplifiers may be required between the cable system headend and an individual subscriber's home.
The use of a television signal comprising amplitude modulated vestigial sideband video subcarriers (AM-VSB) is preferred in the distribution of cable television signals due to the compatibility of that format with the standards of the National Television Systems Committee (NTSC) and the ability to provide an increased number of channels within a given bandwidth. An undesirable characteristic of AM-VSB transmission, however, is that it requires a much higher carrier-to-noise ratio (CNR) than other techniques, such as frequency modulation or digital transmission of video signals. Generally, a CNR of at least 40 dB is necessary to provide clear reception of AM-VSB television signals.
The replacement of coaxial cable with optical fiber transmission lines in television distribution systems has become a high priority. Production single mode fiber can support virtually unlimited bandwidth and has low attenuation. Accordingly, a fiber optic distribution system or a fiber-coax cable hybrid would provide substantially increased performance at a competitive cost as compared to prior art coaxial cable systems.
Amplification of optical signals within an optical fiber network has been a problem in the attempt to distribute AM-VSB television signals. As noted above, amplifiers are required between a cable system headend and a subscriber's home in order to provide signals to the subscriber at an acceptable power level. Semiconductor optical amplifiers of the type typically used in fiber optic systems produce high levels of distortion products that are not compatible with multi-channel AM-VSB video signals. This is due to the short lifetime of the carrier excited state within the semiconductor optical amplifier. The recombination time of such an amplifier operating near 1.3 .mu.m or 1.5 .mu.m is about 1.2 nanoseconds, which is short compared to the period of typical AM-VSB subcarrier operating in the television band of about 55.25 MHz to 1 GHz.
Optical fiber amplifiers, such as erbium-doped fiber amplifiers, have been proposed for applications in long distance transmission and subscriber loop distribution systems. See, e.g., W. I. Way, et al, "Noise Figure of a Gain-Saturated Erbium-Doped Fiber Amplifier Pumped at 980 nm", Optical Amplifiers and Their Applications, 1990 Technical Digest Series, Vol. 13, Conference Edition, Optical Society of America, Aug. 6-8, 1990, Paper TuB3, pp. 134-137, and C. R. Giles, "Propagation of Signal and Noise in Concatenated Erbium-Doped Fiber Optical Amplifiers", Journal of Lightwave Technology, Vol. 9, No. 2, February 1991, pp. 147-154.
The noise figure of the fiber amplifier is a parameter that must be considered in such systems to optimize overall system performance. Noise figures of an erbium-doped fiber amplifier pumped at 980 nm have been found to be near 3 dB, which is a desirable performance figure. However, an erbium-doped fiber amplifier pumped at 980 nm does not exhibit an optimal power efficiency for a communication signal distributed at a typical wavelength of about 1550 nm.
In order to provide a higher power efficiency for a 1550 nm communication signal, erbium-doped fiber amplifiers can be pumped at about 1480 nm. However, pumping at this wavelength results in a noise figure of about 5 dB, which is less than optimal.
It would be advantageous to provide a rare earth-doped fiber amplifier, such as an erbium fiber amplifier, that provides both low noise and high power. The present invention provides such an amplifier.